Masayoshi Son, chairman and CEO of Japan’s Softbank Corp, speaks at a press briefing in Tokyo on Sept. 30.

Softbank Corp. has expanded at breakneck speed since its origins as a software distributor three decades ago manned by CEO Masayoshi Son and two employees. (The two left after Mr. Son told them the company would someday report annual sales of ¥50 billion, fearing for the state of his mind. The company Thursday reported a quarterly net profit triple that and said it expects sales of at least ¥6 trillion in the year to March).

Mr. Son says his corporate group now includes stakes in more than 1,300 companies. But who are they?

Precious little is known about the SoftBank universe. Of the 1,300 companies, SoftBank lists 235 subsidiaries and 108 affiliates as of the end of March. Of these, 150 are consolidated into SoftBank’s earnings reports, and 83 are counted as affiliates or nonconsolidated subsidiaries.

Softbank logs some portion of the earnings of those companies into its profits. But when it comes to the names of the companies and the size of its stakes and investments, SoftBank only disclosed information for 43 as of March.

“That’s it,” a SoftBank spokeswoman said. “That’s all that we disclose. I’m sorry.”

That the empire is growing is clear. In July, Mr. Son closed a $21.6 billion deal to buy a roughly 78% stake in Sprint, then paid another $500 million to raise its Sprint stake above 80%. SoftBank earlier in October announced plans to buy a majority stake in Finland’s Supercell, the developer of the hit tablet game “Clash of Clans,” and a 70% stake in the U.S. firm Brightstar Corp., which handles distribution of smartphones for wireless operators.

These include Yahoo Japan Corp. — 42.5% owned by SoftBank – which has maintained its overwhelming dominance over Google Inc. as Japan’s go-to search engine and portal, even as Yahoo Inc. in the U.S. floundered.

Mr. Son’s firm also owns a majority stake in GungHo Online Entertainment Inc., the developer of the blockbuster mobile game “Puzzle & Dragons.” And SoftBank has also made public details on what may be its most valuable asset: a 36.7% stake in China’s top online retailer, Alibaba Group Holding Ltd. Bankers say Alibaba could list its shares as early as this year in what is expected to be one of the largest initial public offerings by an Internet company.

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Japan Real Time is a newsy, concise guide to what works, what doesn’t and why in the one-time poster child for Asian development, as it struggles to keep pace with faster-growing neighbors while competing with Europe for Michelin-rated restaurants. Drawing on the expertise of The Wall Street Journal and Dow Jones Newswires, the site provides an inside track on business, politics and lifestyle in Japan as it comes to terms with being overtaken by China as the world’s second-biggest economy. You can contact the editors at japanrealtime@wsj.com